The present invention relates to the field of industrial drums and more particularly to the field of tighthead or closed head drums made of thermoplastic materials.
Industrial containers made of thermoplastic materials are well known for the transportation and storage of materials, including hazardous materials in liquid and powder form. Such drums are generally categorized as open head drums, where the top of the drum comprises a removable lid attached to the drum by a tension ring, and closed head or tighthead drums where the top is integrally formed with the drum.
All such drums are regulated by the United States Department of Transportation which requires that the drums perform to certain minimum standards in a variety of physical tests. One such test is a xe2x80x9cdropxe2x80x9d test where a drum full of a liquid is dropped on its side from a predetermined height.
Moreover, both open top and tighthead drums experience challenges when the full drum needs to be moved since a full 55 gallon drum can weigh up to 880 pounds. As such, it is preferred that industrial drums include a handling ring which can be engaged by a mechanical handling means such as a parrot beak, which is well known in the art.
With specific respect to plastic tighthead drums, early versions comprised separately formed handling rings which were subsequently attached to the drum by a compression fit. Later innovations allowed for the manufacture of plastic drums with integrally formed handling rings. The integral handling rings were formed by compression molding within the blow mold during the blow molding process.
However, these designs each suffered inadequacies during drop testing to determine impact resistance, due to the geometry in the area where the handling ring was formed on the drum body. More particularly, plastic drums with integrally molded handling rings generally have a section thicker than the drum body at the intersection of the handling ring and the drum body. This section is therefore often uneven and contains stress points that contribute to failure during mandatory drop testing, which are required to be conducted at low temperatures.
It has been found that the thicker sections provided places where the polyethylene or other polymeric thermoplastic material formed weak crystalline structures due to the slow cooling in that area. Such crystalline structures do not adequately resist breakage during drop impact resistance testing at low temperatures, i.e., at 0xc2x0 F. Additionally, the irregularly formed extrudate in the area of the intersection of the handling ring and the drum body provides points at which the stresses inherent during drop impact create an initiation point for the cleavage of the crystalline structure, causing failure.
Various configurations of handling rings and the area of intersection of the handling ring and the drum bodies have been developed in a number of attempts to avoid failure in the area of the handling ring during drop testing, however, improvement is needed.
It is therefore an object of the present invention to provide a thermoplastic closed head drum including an integrally formed handling ring with improved resistance to failure in the area of attachment of the handling ring during drop testing.
This and other objects are achieved by the present invention which is directed to a one piece thermoplastic tighthead drum comprising a generally cylindrical body, an integral bottom at the lower end of the body and an integral top at the upper end of the body. At least one integral handling ring is included which extends from the drum at the area in which the drum body merges with the at least one of the top and/or bottom, referred to herein as the area of merger.
The top and/or bottom of the drum having the handling ring associated with it includes a raised central section of the top or a lowered central portion of the bottom, commonly referred to herein as a raised central portion, surrounded by an annular groove. The annular groove is defined by a substantially flat or horizontal floor, a first sidewall connecting the floor to the raised central portion and a second sidewall comprising at least a portion of the handling ring extending from the drum body at the groove floor. Thus, the handling ring extends from the drum at the area where the drum body converges with the top and/or bottom at the outer perimeter of the annular groove to form the area of merger between the body, the top and/or bottom having the handling ring associated with it and the handling ring.
The interior surface of the drum in the area related to the convergence or merger of the drum body, the annular groove and the handling ring is formed without sharp changes in direction or notches. The inner surface in the area of merger is manufactured in the form of an undulating surface comprising four (4) radii, each of which relate to a change of direction along the interior surface of less than 90xc2x0. When the handling ring is extending upwardly, either at the drum top when the drum is right side up or at the drum bottom when the drum is upside down, the first radius turns inward of the surface in an arc from the upper sidewall of the drum body to the bottom of the handling ring. The second radius turns upward from the first radius in an arc from the bottom of the handling ring to generally under the side of the handling ring. The third radius turns inward from the second radius in an arc from generally under the side of the handling ring to below the floor of the annular groove. The fourth radius turns upward from the third radius in an arc from below the floor of the annular groove to the inside of the first sidewall of the annular groove. The interior surface then continues along the interior of the first sidewall and to a smooth radius where the first sidewall meets the raised central section.
This configuration of a smooth undulating inner surface in relation to the sidewall, handling ring and top and/or bottom followed by the change in direction from the first sidewall to the raised central section provides a double hinge above the area where the handling ring merges with the drum body, traditionally the area most prone to failure during drop impact. The double hinge absorbs the energy of the impact during drop conditions thus preventing stresses from pulling or bending in the critical weld area where the handling ring merges with the drum body.
The configuration of the interior surface at the area of merger, including the obtuse radii angles, is preferably formed by the excess material or extrudate forced out of the handling ring as it is compression molded. The extrudate in the present invention is adapted to create the interior profile in as close to a linear formation as possible in the area of merger of the drum sidewall handling ring and the top of the drum.